When a radio frequency (RF) signal propagates from a transmitter (TX) through a transmission media to the receiver (RX), the signal strength is reduced due to path loss. The path loss is proportional to fL, where f is the frequency of the signal and L is an environment-dependent parameter. In a practical non-line-of-sight (NLOS) environment, the parameter L is normally between 2 and 4. This physical law suggests that the higher the operating frequency is, the higher its path loss is within the same operating range. This means that more energy per information bit has to be radiated, compared to a lower frequency band.
One characteristic of high frequency bands, such as an extremely high frequency (EHF) band ranging from 30 GHz to 300 GHz, is that atmospheric loss is substantial. To overcome extra path loss on high frequency bands, traditionally the prior art EHF communication systems use directional antennas, such as horn antennas, with substantial antenna gains in both transmission and reception. Another approach was to use multiple antenna arrays and beam-forming techniques to form directional beams between the transmitter and receiver.
While high-gain directional antennas work well for fixed point-to-point communications, they create problems when the stations or nodes are portable or mobile. The hidden node/network is a typical problem, where a network is composed of two or more connected nodes. In the prior art, a node normally scans/tests the channel on the operating frequency band immediately after it is powered on. For example, a new node intending to use to a channel in the EHF band will tune its receiver to the EHF channels and determine the channel status (free or occupied) by analyzing the signals received.
If a node/network is detected, the new node may decide to join the existing node/network or initiate a new network on another free channel. However, the fact that both the transmitter and receiver antennas are highly directional means that the new node can only detect an existing node/network if their antenna directions happen to be aligned by chance. As a result, the probability of missing the detection of an existing node/network is very high. If a new node misses the detection of the existing nodes on the channel, the new node will either miss the opportunity to join the existing nodes or the new node will use the channel and cause interference for the others. Interference between existing nodes can also happen. For example, a receiver receiving data from a transmitter can be disturbed by an another transmitter of a 3rd node, as the 3rd node cannot hear the communication between the first two nodes and assumes the first two nodes are idle.
To establish and maintain a network on the EHF channel under these traditional scenarios, many management messages have to be exchanged. Because each node in the network uses highly directional antennas, effective broadcast and multicast schemes cannot be applied. One solution is to let the transmitter and receiver repeatedly transmit and receive at all directions for a long time, to increase the chance of antenna direction alignment. This solution is however not power efficient. Another problem is that it is very hard for a new node to become synchronized to the existing nodes in time and frequency.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.